I don’t think we will ever see TouchID again in the premium devices.
Assuming FaceID has no major issues (and I expect it will be fine) then there is no need at all for Touch ID. Especially as in the Keynote it was made clear hay FaceID is 20 Times more secure - why would they go backwards?
Whilst some people think that the removal of TouchID was purely down to technical limitations putting under screen - I believe that FaceID was always seen as the future (replacing TouchID), but they wanted to still include TouchID whilst transitioning and to buy them more time in implementing the face authentication. The technical issues simply accelerated their plan.